| Summary: | 碩士 === 國立高雄海洋科技大學 === 輪機工程研究所 === 99 === This study examines the performance of heat transfer enhancement (THE) for piston cooling system in marine propulsive diesel engine. The HTE effect of a reciprocation blind duct with a lateral entry jet is investigated. The detailed heat transfer measurements over the jet and back walls of the static and reciprocating blind duct at Re(Rej) of 1500(6000), 2000(8000), 3000(12000), 5000(20000), 7000(28000) and 10000(40000) with the reciprocating frequencies of 0, 0.67, 0.83, 1 and 1.17 Hz are performed. For revealing the flow structures in the vertical blind duct with a lateral entry jet, the turbulent flow and heat transfer characteristics in the static blind are analyzed using the commercial CFD STAR CD code. The simulation conditions for this numerical analysis are Re(Rej)=5000(20000), 7000(28000) and 10000(40000). The detailed Nusselt number distributions over the jet, back, impingement, side, and end walls of the static blind duct are evaluated numerically. By adjusting the heat flux over each constituent duct wall to vary the gravitational Grashof number (Grg) during the numerical analysis, the buoyancy effect on the heat transfer performances in the simulated blind duct is examined. The inter-correlations between the heat transfer distributions over all the constituent duct walls and the flow structures developed within the static duct are numerically examined. Based on the numerical results, the variations of the area-averaged Nusselt number ( ) over each duct wall against Re and Grg are analyzed. Relative to the Dittus-Boelter correlation heat transfer level (Nu∞) for the developed turbulent flow in smooth pipe, the calculated ratios of / Nu∞ for the jet, back, impingement, side and end walls of the static blind duct are raised to 3-5.7, 2.8-5.6, 3-5.8, 2.7-4.8 and 3.1-6.1, respectively. These / Nu∞ ratios decrease as Re increases.
The experiments are performed at the steady states for the static duct and at the quasi-steady state for the reciprocating duct. The heat transfer measurements form the static and reciprocating test ducts are compared to highlight the influences of duct reciprocation on the heat transfer performances. By examining the detailed Nusselt number distributions over the jet and back walls obtained at the various test conditions, the effects of Re (Reynolds number), Pu (Pulsating number) and Grp (Reciprocating Grashof number) on the heat transfer properties over the jet and back walls of the blind duct are revealed. The following parametric analysis examines the impacts of Re, Pu and Grp on the area-averaged Nusselt number over the reciprocating duct wall ( ) and the pressured drop coefficient (f) with the attempt to generate the and f corrections. Acting by the combined Re, Pu and Grp effects, the ratios of / calculated from the experimental data collected from the jet and back walls of the blind duct are in the respective ranges of 0.78-1.3 and 0.8-1.51.
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